This invention relates generally to methods and apparatus for determining flow rates of fluid in a pipe or well, and more specifically, to methods and apparatus for determining the flow rates of individual single phases of the total flow rate of a multi-phase fluid flow mixture in a wellbore.
In producing wells it is common to find the well fluid consisting of multiple phases, such as oil and water, oil and gas, or oil, water and gas. Often, one or more of such phases is an undesirable part of the well production. For example, in the case of a flow regime comprising oil and water, as is common in such producing wells, the oil is the fluid phase desired to be produced while the water phase is typically an undesirable portion in the production flow. In a well which produces oil and water phases, logging surveys often are made for the purpose of establishing the flow profile. This profile indicates the rates at which oil and water are produced from each formation interval of interest. A profile is desired which accurately indicates those intervals producing oil, those producing water, and those producing both fluids. Such a profile is indispensable for improving the oil production, reducing the water phase production, and characterizing and managing the reservoir.
For flow profiling, it is necessary to establish, at a desired depth, the total flow rate of the oil and water phases, as well as the water cut, which is the water flow rate expressed as a fraction of the total flow rate. The logging instruments used in the survey typically produce responses from which the total flow rate and the water holdup can be derived. Water holdup is defined as the volumetric concentration of water in the well bore. In order to generate the flow profile, it is necessary to relate the water cut to the water holdup and the total flow rate at any depth of the well.
In customary practice, water cut is derived from a relationship which involves the water holdup, the total flow rate, and the slip velocity, which is the amount by which the flow velocity of the oil phase exceeds the velocity of the water phase due to the difference between the fluids' densities. There are a number of difficulties attendant to this method of deriving the water cut:
1. Since slip velocity cannot be measured in the well survey, it must be estimated independently; PA0 2. Charts for estimating the slip velocity usually pertain to bubble flow, i.e., oil bubbles percolating through water. These charts should not be used in the many instances when the flow regime cannot be characterized as bubble flow; and PA0 3. Available charts and correlations for slip velocity pertain to vertical bubble flow, and should not be relied upon when the oil-water flow is in a deviated well bore.
Accordingly, the present invention presents a new method and apparatus of relating the water cut to the total flow rate and the water holdup. The method is applicable in both vertical and deviated well bores, and it is not limited to a bubble flow regime. The method is based on a set of predicted response curves for fluid density and fluid capacitance instruments in oil-water flows. These curves are derived from measurements recorded in a test flow loop prior to determining the flow rates of the oil-water regime in the borehole.